1. Field of the Invention
The present invention relates to the field of photorepeaters used, in the fabrication of semiconductor circuits, to expose several regions of an integrated circuit fabrication wafer according to a determined set of patterns. The present invention applies, more specifically, to a method for characterizing a photorepeater.
2. Discussion of the Related Art
FIGS. 1 very schematically illustrates, in cross-section, the operating principle of a photorepeater.
A photorepeater is essentially formed of a light source 1 for successively exposing regions 2, 2', and 2" corresponding to chips of integrated circuits to be made on a substrate 3, for example a silicon wafer. For example, the photorepeater is used to define patterns of an etching or deposition mask, implemented in a resist layer 4 previously deposited on substrate 3. The patterns to be reproduced on each chip 2, 2', and 2" are defined by a reticle 5 implemented based on the data of conception of the integrated circuit. Focusing lenses 6 and 7 are respectively placed between light source 1 and reticle 5 and between reticle 5 and the region of substrate 3 to be insulated. The different regions of substrate 3 to be insulated are successively placed directly below light source 1. In FIG. 1, the movement of a silicon wafer has been symbolized by an arrow F, region 2' being illuminated, while region 2" has already been exposing and region 2 is waiting. The patterns, symbolized by holes 8 in reticle 5, are reduced by lens 7 to form exposing areas 9.
A problem which arises upon use of a photorepeater relates to maintaining the pattern dimensions in the final mask performed on each chip of the integrated circuit. The dimension of these patterns depends, in particular, on the sensitivity of resist 4, on the distance between the plane of the wafer and the image plane of the reticle, and on the illumination dose, that is, on the time for which a same region is exposing and on the intensity of the radiation that it receives. This problem of pattern reproducibility is even more acute in the frequent cases where several photorepeaters are used to fabricate, in large series, identical integrated circuits. In such cases, the same mask has to be reproduced on each region 2, no matter which photorepeater is used.
It is generally desired, in particular, to guarantee an identical illumination dose for each photorepeater.
It is acknowledged that by setting each photorepeater on an identical nominal illumination dose, important variations on the dimensions of the patterns reproduced by the different photorepeaters may result.
A first conventional solution to solve this lack of reproducibility consists of measuring, by means of a photosensitive cell generally associated with each photorepeater, the illumination dose for a given order so as to calibrate, individually, each photorepeater. A disadvantage of such a solution is that the photosensitive cells can themselves exhibit drifts from one photorepeater to another and thus do not guarantee that the illumination doses will be identical for all photorepeaters.
A second conventional solution consists of using a standard photosensitive cell that is displaced from one photorepeater to another to calibrate each photorepeater based on the illumination dose measured by this single sensor. Such a solution is not satisfactory because, like the former solution, it measures the general illumination of the region insulated by the photorepeater and does not take into account drifts of other parameters, in particular, the resist laying and development conditions and the optical aberrations of the lens, notably at its circumference.
A third conventional solution consists of making, with each photorepeater, a test wafer based on the reticle which is associated with the photorepeater and which defines the patterns of the integrated circuit to be fabricated. The illumination dose is varied from one region of the plate to another. Then, the dimensions of a reference pattern, generally provided at the edge of the reticle (outside the patterns defining the integrated circuit) are measured. The dimension of the reference pattern generally corresponds to the critical pattern (the smallest) of the mask to be implemented, for example, 0.35 .mu.m, 0.5 .mu.m, or 0.7 .mu.m. Then, based on these measurements, the illumination dose to be applied at the series production is set. A disadvantage of such a solution is that the reference pattern generally is placed at the edge of the pattern to be reproduced, that is, where there are the most optical aberrations. Another disadvantage of this solution is that it has to be repeated for each new fabrication series.
As for the previous solutions, the implementation of such a solution results, in practice, in dimensional drifts of about 20% from one photorepeater to another.
The difficulties encountered to guarantee the reproducibility of a same pattern even though several different photorepeaters are used generally compel the use of a single photorepeater per integrated circuit production series, which reduces the output of a production line.
Further, even by using a single photorepeater, conventional solutions, for measuring the illumination doses or checking the dimensions by means of a reference pattern on the chips, do not avoid significant dimensional drifts of the reproductions, on the same chip, of the same patterns appearing several times on the same reticle.